Pre-BCR Signaling Activity Predicts Sensitivity To Syk Kinase Inhibition In B Cell Acute Lymphoblastic Leukemia (B-ALL)

B lymphocyte development proceeds in a stepwise fashion and is tightly linked to the generation of a functional B cell receptor (BCR). At the preB cell stage B lymphocyte progenitors express the precursor B cell receptor (pre-BCR), an immature form of the BCR consisting of two µ heavy chains (µHC) and two surrogate light chains (SLC). Pre-BCR expression marks the proB to preB transition and induces a burst in preB lymphocyte proliferation. In 20% of the cases B cell acute lymphoblastic leukemia (B-ALL) arises from lymphocytes arrested at the pre-BCR positive stage of lymphocyte development (preB-ALL). Due to the essential role of the pre-BCR for preB cell proliferation we hypothesized that pre-BCR signaling also is involved in the maintenance of preB-ALL. Consequently, pharmacological inhibition of Spleen tyrosine kinase (Syk), the main transducer of pre-BCR signaling, may serve as effective treatment for this subtype of B-ALL.

We analyzed a panel of six ALL cell lines (SMS-SB, RCH-ACV, Nalm-6, Kasumi-2, 697, KOPN-8) arrested at the pre-BCR⁺ stage of B lymphocyte development (cytoIgµ⁺, sIgM^-). Assessment of the baseline phosphorylation levels of the pre-BCR associated kinases Lyn, Syk and Btk by immunoblotting and subsequent densitometric analysis allowed us to assign B-ALL cells into groups with either high levels of Lyn, Syk and Btk phosphorylation or with low or absent phosphorylation of these kinases, respectively. Moreover cell lines with highly phosphorylated Lyn, Syk and Btk also exhibited lower surface pre-BCR expression than cell lines with low phosphorylation levels. As pre-BCR activation is followed by its rapid internalization the concomitant presence of low pre-BCR expression and high phosphorylation of pre-BCR associated proteins suggests increased pre-BCR pathway activity.

When we investigated the impact of pharmacological inhibition of the pre-BCR associated kinase Syk through the highly specific inhibitor PRT060318, preB-ALL cell lines with highly phosphorylated pre-BCR associated molecules turned out to be more sensitive to Syk inhibition (IC₅₀ < 1.6µM) than preB-ALL cell lines with less phosphorylation (IC₅₀ > 3.9µM). In proliferation assays PRT060318 inhibited preB-ALL proliferation in a dose dependent manner, whereas PRT060318 did not induce apoptosis in concentrations as high as 5µM. This supports the notion that pre-BCR signaling activity may be more relevant for preB-ALL proliferation than for preB-ALL viability. In line with these results the pre-BCR^- proB-ALL cell lines REH and RS4;11 were highly resistant to Syk inhibition in all functional assays (IC₅₀ > 10µM), suggesting that pre-BCR expression is a prerequisite for sensitivity to Syk inhibition.

To examine the molecular changes following pre-BCR inhibition, ALL cells were treated with increasing concentrations of PRT060318 (100nM-5µM) for two hours and then subjected to immunoblotting. Syk inhibition led to a dose dependent decrease in AKT phosphorylation in all preB-ALL cell lines and subsequently reduced phosphorylation of FOXO transcription factors. In the resistant proB-ALL cell line REH, AKT and FOXO phosphorylation were not affected.

Gene expression analysis of the preB-ALL cell lines RCH-ACV and Nalm-6 further suggested that PRT060318 interferes with pre-BCR signaling. Treatment with 1µM PRT060318 for 72h reduced the expression of genes associated with pre-BCR signaling (e.g. BCL6, CD22, PTPN6) and Ingenuity Pathway Analysis identified pre-BCR signaling as the main target of PRT060318 in both cell lines (p<0.05). We are currently validating the GEP analysis by quantitative PCR and immunoblotting.

In conclusion, we provide evidence for the efficacy of Syk inhibition in pre-BCR⁺ ALL. Moreover we were able to correlate the baseline phosphorylation status of pre-BCR associated proteins and pre-BCR expression levels with the sensitivity of preB-ALL to the Syk inhibitor PRT060318. These findings provide a first rationale for the clinical testing of Syk inhibitors in preB-ALL, and suggest that activation status of pre-BCR associated molecules can help in selecting preB-ALL cases that are particularly sensitive to Syk inhibition.